Certain calcium ion (Ca2+) channels, e.g., voltage-gated (VGCC) and transient receptor potential (TRP) Ca2+ channels, are highly expressed in some cancers, suggesting that they could be therapeutic targets. Indeed, drugs specifically targeting subclasses of these channels already exist and two are currently in Phase I clinical trials, but it is difficult to unravel which specific channel to target in individual tumors. Our lng-term goal is to develop an analytical, non-invasive assay of tumor calcium channel activity in vivo that could eventually be used clinically as a means to individualize Ca2+ channel-targeted therapies. To achieve this goal, we will measure tumor uptake of manganese ion (Mn2+), an MRI contrast agent that largely enters cells via Ca2+ channels. Our preliminary studies show that on manganese-enhanced MRI (MEMRI) examination, different types of human cancer cells take up exogenous Mn2+ in a calcium channel dependent fashion, resulting in an increase in MRI longitudinal relaxation rates, i.e., R1 values. In addition, specific and systemically administered agonists and antagonists of VGCC channels modify non-tumor tissue Mn2+ accumulation in vivo. The overall hypothesis of this proposal is that the extent of tumor uptake of Mn2+ in the presence of Ca2+ channel-specific agonists or antagonists, measured using manganese enhanced MRI (MEMRI), is a useful biomarker of specific calcium channel activity in the tumor and thus a rational approach for predicting potential tumor therapeutic response to Ca2+ channel-based therapies. To fully test this hypothesis, we will address two specific aims. In Aim 1, we will define the sensitivity of MEMRI and specific Ca2+ channel modulation as a method to identify the predominant Ca2+ channels expressed in different types of human tumor cells in vitro. We will characterize the prevalent Ca2+ channels in various human tumor cell lines using Western blots, treat the different cells with agonists and antagonists to specific Ca2+ channels, and then measure Mn2+ uptake in each cell line using MEMRI. The prediction is that specific agonists and/or antagonists will change the cellular R1 values, thereby identifying the key channel(s) expressed by the specific tumor cells. In Aim 2, we will determine if the combination of MEMRI and specific Ca2+ channel modifiers can identify dominant Ca2+ channel types in different human tumor xenografts in vivo. Based on the in vitro studies, selected agonists and antagonists will be injected into SCID mice bearing subcutaneous human xenografts, and average R1 values will be determined in the tumors following MnCl2 injection. The prediction is that injection of specific agonists and/or antagonists will change the tumor R1 values, thereby identifying the key Ca2+ channel(s) expressed by the specific xenografts. We envision that the combined use of MEMRI and modulation of specific Ca2+ channel activity will eventually be useful to noninvasively determine whether a targeted Ca2+ channel is expressed in a patient's tumor, thereby guiding subsequent Ca2+ channel-based treatment.